MgCO{11 {11 ADDITION TO CaSO{11 {11 CONTAINING SEA WATER TO PREVENT CORROSION

ABSTRACT

A process for evaporating sea water without causing corrosion of the steel materials of the apparatus comprising reacting calcium sulfate dissolved in the sea water with magnesium carbonate to such an extent that 5 to 10 percent of the original Ca remains dissolved while the formed calcium carbonate is removed by filtration, adjusting the pH of the treated sea water with a mineral acid to pH 5.0 to 5.5, deaerating it with the rate of deaeration exceeding 90 percent and then introducing the sea water into a stage of a flash evaporation apparatus the temperature of which is lower than 80*C.

IUiiit ties tet n91 Tahata et all.

[ MGCOs ADDITION TO CASO4 CONTAINING SEA WATER TO PREVENT CORROSION [76]Inventors: ll-Iisanolmu Talbata; Norirnasa Tahata,

both of No. 968, Kou, Ohgoshi-machi; Eollruro Nalkaiima, No. 1831Ohyabu-cho, all of Sakaicle-shi, Japan [22] Filed: Dec. 3, 11970 [21]Appl. No.: 94,726

[30] Foreign Application Priority Data Dec. 15, 1969 Japan 44/100773[52] US. C1. 203/7, 203/11, 202/173, 252/175 [51] Int. Cl. 180M 3 /06[58] Field oi Search 203/7, 10, 11;

252/80, 88, 175, 180; 23/66, 304, 312, 312 W; 159/DIG. 13; 202/173, 174

[ 1 Oct. M, 11973 Primary Examiner-Norman Yudlcoff AssistantExaminerDavid Edwards Attorney-McGlew & Toren [57] AESTEACT A processfor evaporating sea water without causing corrosion of the steelmaterials of the apparatus comprising reacting calcium sulfate dissolvedin the sea water with magnesium carbonate to such an extent that 5 to 10percent of the original Ca remains dissolved while the formed calciumcarbonate is removed by filtration, adjusting the pH of the treated seawater with a mineral acid to pH 5.0 to 5.5, deaerating it with the rateof deaeration exceeding 90 percent and then introducing the sea waterinto a stage of a flash evaporation apparatus the temperature of whichis lower than 80C.

4 Claims, 5 Drawing Figures Fresh Water Patented Oct. 16,1973 3,766,019

4 Shasta-Sheet '1 Effect of H on Corrosion 01 Apparatus Steel Materials400C 22nc 0.225

Corrosion Rate yeur) Generation 0.075 of Hydrogen INVENTOR m? 0 BYROHURO NANAIIMA amo 7mm ATTORNEYS Palenled Ucl. l6, ll973 3J6UH 4swam-5mm :1

FlGQ

Relation Belween NaCl Concenlralion and Corrosion Rate of ApparatusSteel (carbon sleeUMaterials at 35C Corrosion Rate 50 d mg/drn per 218')i ConcenlralionlN) Relation Between Dissolved CO2 and Corrosion Rate ofApparatus Steel Materials 0150 at60C Velocity of 5 lmin Corrosion Rate25 /year) O 5 l0 2O CO2 Concentration /l) BY RUNURA NAwmnMA wWIMWATTUIYIVEYS Patented Uct. flfi, 1973 Fresh Water 4 ShwMs-SMM 4.

2 c c o .0 L. u U

E .2 U] m c m U E 0 Water WCTTOR NEYS MGCU AtlDlDtll'llllUN TU CAM)CONTAHNIING SEA WATER T lPllklEl/IENT CUWROMON The present inventionrelates to a process for evaporating sea water with almost completeavoidance of corrosion of the steel materials on the flash evaporationapparatus.

At present, sea water is considered the most promising source for makingfresh water to meet the shortage of water for various uses. Amongmethods to make fresh water from sea water, the evaporation method drewparticular attention due to the simple apparatus required, easiness ofoperation and the certainty of production, and for these reasons,attempts have been made to develop the method.

The cost required in making fresh water by evaporating sea water couldbe reduced by reduction of the cost of the equipment and heat source andby utilizing the by-products. These inevitably should be the principalproblems in the developing investigation.-

The cost for equipment occupies approximately 40 percent of the totalcost of making fresh water and especially the cost of the heatconducting materials amounts to about the half of the cost of equipment.

As has been mentioned above, the evaporation method for making freshwater from sea water is characterized by simple equipment and operation,and therefore the best way to reduce the cost for making fresh water isto reduce the cost of the equipment which is generally recognized.Moreover, reduction of the cost of the heat conducting materials whichoccupies about 25 percent of the cost of the equipment is the mostimportant.

The present invention is particularly concerned with this point andcomprises a process for the flash evaporation of sea water withoutcausing corrosion of the steel material of the apparatus. It is anepoch-making process in which ordinary steel materials, welded or drawn,may be used as the steel materials for the apparatus while avoiding mostof the usual precautionary measures against corrosion.

So far, no specifically recommended materials have been found for use asa heat-conducting material for the flash evaporation apparatus in makingfresh water by evaporating sea water, and such materials are currentlybeing sought in development research. In fact, however, almost all ofthe current flash evaporation apparatus employ copper alloys for tubematerial considering the cost for the material.

Prices of various heat conducting materials, which while not used forevaporating sea water, are used in other various apparatus, are shownfor the sake of comparison.

Material Ratio of price oftubes Carbon steel (Welded tube) 0.8

Carbon steel (Seamless) Stainless Steel (SUS 27) Stainless steel (SU 28)25 Cr-ZO Ni steel lncoloy alloy 800 Heat resisting cast steel (HU type)Al-Ni Monel metal Heat resisting aluminum Copper Glass lining EpoxyresinCarbon steel with rubber lining The present inventors have developed anexcellent process for flash evaporating sea water in which not onlyexpensive alloy steels but also steel tubes that have almost never beenused due to corrosion could be used as the material for the heatconducting tubes in the flash evaporating apparatus.

More particularly, the present invention is a process for evaporatingsea water without causing corrosion of the materials of the apparatus,comprising reacting magnesium carbonate with calcium sulfate dissolvedin the sea water to such an extent that Ca ion remains in the amount of5 to 10 percent of the sea water, filtering off the formed precipitateof calcium carbonate, adjusting the pH of the filtrate to 5.0 to 5.5with a mineral acid, deaerating it with more than 90 percent deaerationand introducing it into a stage less than C of a flash evaporationapparatus which is at a temperature of less than 80C.

In other words, the present invention has permitted the flashevaporation of sea water without almost any corroding of the steelmaterials of the apparatus by avoiding the corroding environment usuallypresent, as well as by forming corrosion resistant films on the surfaceof steel materials. This is accomplished by using conditions oftreatment which are adequately adjusted, such as, the condition ofdecalcium treatment where calcium sulfate dissolved in sea water wasreacted with magnesium carbonate, the condition to adjust the pH, thecondition of deaeration and the condition under which the sea waterafter the treatment was brought into the flash evaporation apparatus.

The following condition should be preferably selected in the presentinvention when a precipitate of calcium carbonate is formed by reactingmagnesium carbonate with calcium sulfate dissolved in sea water.

The reaction should be preferably carried out at a temperature higherthan 40C under stirring. The reaction normally more than 4 hours at 40Cand about 3 hours at 60C. In this case, the ratio of CaSO, to MgCO inmolar ratio should be larger than 1 l, and the MgCO should be preferablyused immediately after preparation and the ratio in weight of CO to MgOin the MgCO be i 1. When the treatment is carried out under theconditions mentioned, to percent of the Ca dissolved in sea water isprecipitated in the form of CaCO and approximately 5 to 10 percent ofthe ion remains in the sea water, which is presumably in the form ofCaCO or CaSO in the case where an excess of magnesium carbonate isadded, the excessive amount of magnesium carbonate is precipitated andfiltered off along with the deposited calcium carbonate.

In this treatment, 90 to 95 percent completion of the reaction isfavorable in the operation and is a necessary factor to form a corrosionpreventing film. For the reaction to proceed over 95 percent, a longertime is required and even when the temperature is raised over 60C, thisis hardly effective to form a corrosion preventing film.

Sulfuric or hydrochloric acid isused to adjust the pH of the sea waterto 5.0 to 5.5 after the decalcium treatment described above. Theaddition of hydrochloric acid results in a better thermal efficiency andlarger elevation of boiling point of the solution after the pHadjustment than the addition of sulfuric acid, but sulfuric acid canattain the same object, though to a slightly lesser extent. Thereforeuse of sulfuric acid is economically favorable.

Deaeration is conducted in the present invention at the temperature ofsea water, i.e., 45 to 55C with a deaeration rate of 90 to 95 percent.The preferred condition of deaeration is useful in the operation andproduces good conditions for the succeeding stages, as described in thefollowing paragraphs.

The rate of deaeration is related to both temperature and degree ofvacuum. Thus, a high temperature and a high degree of vacuum arefavorable to obtain a high rate of deaeration, but at the hightemperature and at the high degree of vacuum, evaporation of the liquidis enchanced and the vapor produced must be suctioned with a vacuumapparatus such as steam ejector. This is unfavorable in that suction ofthe non-condensing gas is inhibited, the vacuum apparatus occupies alarge space and the amount of the steam required for operation becomeslarge. For high vacuum, in addition, more stages are required for thesteam ejector which necessarily results in a large steam requirement forthe operation. In considering these factors, the favorable degree ofvacuum should be approximately 40 Torr and the rate of deaeration atthis degree of vacuum should be about 90 to 95 percent for sea water at40 to 55C.

In the treatment above, however, 5 to percent of CO remains in the seawater. When the pre-treated sea water is introduced into the flashevaporation apparatus described below wherein the temperature is below80C, deaeration as well as evaporation occurs. Deposition of CaCO occursin the meantime which causes elevation of the pH of the sea water and atthe same time, a fraction of the deposit presumably forms a corrosionpreventing coating film when deposited on the surface of apparatus. Thedeposited particles of CaCO are so tiny that they can hardly be observedwith the naked eye. Therefore, they have almost no influence on thethermal conductivity of the materials, as was confirmed.

The deaerated sea water in the manner described above is subsequentlyintroduced into a flash evaporation apparatus under the condition shownbelow. In this case, sea water at a temperature above 80C has littleinfluence on the corrosion preventing effect, but a precipitate ofmagnesium hydroxide is usually formed which is a factor in the scaleproblem.

In fact, the deaerated sea water is introduced into the flashevaporation apparatus at a temperature below 80C for reasons of whichwill be explained as follows.

In the flash evaporation apparatus, the stages which are at temperaturesbelow 80C are the so-called heat rejection part of the apparatus andtherefore the fluid in the heat conducting pipes is a cooled sea waterfor which the pre-treated sea water described previously is used.Namely, the pre-treated sea water is cooled with a cooling tower and iscirculated again. This statement applies around the 18th stage of theflash evaporation apparatus if it contains stages. The temperature ofthe 3rd stage from the last is below 80C at which unfavorableprecipitation of Mg(OH) begins to be formed. The degree of vacuumgradually changes, so that, for example, it is 110 Torr at the l8th, 90Torr at 'the 19th, and 50 Torr at the 20th stage. The temperature islowered in parallel. Therefore evaporation occurs in the 3 stages, wherethe non-condensing gas remaining unexpelled in the deaerator is almostcompletely deaerated along with the evaporation of water which occurs toa great extent in these stages. The non-condensing gas dearated in thesestages is expelled immediately from the system passing through thecooling part attached to the l8th, 19th and 20th stages. Thus, a factorwhich can cause corrosion can be effectively avoided.

More particularly, a feature of this process is the immediate removal offreed corrosive gases such as 0 and CO which are not deaerated in thedeaerator described above. Conditions for the complete dearation are (l)a long time of deaeration, (2) tiny particles of the liquid and (3) ahigh degree of vacuum. Condition (1) is best satisfied in the upperstages, but the scale trouble caused by the precipitation of Mg(Ol-I) ismore likely to occur in the upper stages where the temperature ishigher.

Condition (2) means a large exposed surface area which, however, doesnot postulate the 18th stage.

Condition (3) is satisfied mainly at the last stage since the degree ofvacuum is the highest at the last stage, but condition (1) is notsatisfied there. Another difficulty is that, if the pre-treated seawater which is usually near 50C is introduced into the last stage whichis near 35C, evaporation becomes more intense in the last stage than inthe other stages or the water is taken out of the system without beingevaporated. Either case is unfavorable.

The purpose of the evaporation step is not attained by the processmentioned and at the same time evaporation as well as deaeration is notcarried actually effected.

Another unfavorable factor is introduction of a high temperature liquidinto the last stage which should be cooled. This leads to the neglecteddifference in the effective temperatures and the lowered thermalefficiency and also to unstable operation.

The present invention will be explained in more detail along withvarious phenomena which take place in the process of this inventionreferring to the attached drawings in which:

FIGS. 1 to 3 are explanatory of corrosion occurring on steel materialsof apparatus in the flash evaporation of sea water in accordance of thisinvention. More particularly,

FIG. 1 is explanatory of the effect of pH on the corrosion of the steelmaterials, and

FIG. 2 shows the relation between the concentration of NaCl and the rateof corrosion which occurs on the steel materials of apparatus, and

FIG. 3 shows the relation between the concentration of CO and the rateof corrosion which occurs on the steel materials of apparatus.

FIG. 4 explains the usually employed process of flash evaporation andFIG. 5 explains the apparatus which appears in the examples of thisinvention.

First, corrosion of steel materials of the apparatus to be used in thepresent invention will be mentioned in relation with pH. In general thesurface of steel materials is covered by a coating film of Fe(Ol-I)between a pH 5.0 and 9.5 so that the degree of corrosion is independentof pH of the water as shown in FIG. 1. Therefore the rate of corrosiondepends in this range of pH on the amount of oxygen dissolved in thewater. At a pH above 9.5, the solubility of the coating film of Fe(OH)is decreased and at the same time iron is converted into a passive stateand, as a result, corrosion ceases to occur almost completely. At a pHbelow 4.0,

lFe(Ol-I) is dissolved and the coating film is removed, and thereforehydrogen evolves and the rate of corrosion is increased. The evolutionof hydrogen is limited to a pH below 5 where the influence of dissolvedoxygen is considerable. The relation between pH and corrosion is variedwhen the solution contains different salts which attack the coating filmof Fe(OI-I) For example, in a solution containing equivalent amounts ofCl ion and H CO corrosion occurs vigorously even if the solution isneutral or alkaline.

Second, the relation between the amount of dissolved oxygen and thecorrosion of steel materials of the apparatus of this invention will bementioned.

The amount of dissolved oxygen is decreased with the increase in theconcentration and the temperature of the sea water. The concentration ofoxygen is a factor in determining the rate of corrosion.

The dissolved oxygen acts as depolarizer to accelerate corrosion atconcentration up to ppm, while at concentrations above 10 ppm it acts assuppressor by forming protective films on the surface of iron. When theconcentration of Cl ion is large, however, the protective films becomeunstable and therefore the suppressive effect of oxygen disappears. Inthe sea water which is slightly alkaline (pH 8.0), the rate of corrosionis increased with the increase of dissolved oxygen up to about 16 ppm,but is no longer increased at a larger concentration of oxygen. Inneutral sea water (pH 7.0), the amount of corrosion is increased foroxygen concentration up to 16 to 20 ppm, while it is decreased forlarger concentrations. In slightly acid sea water (pl-I 6.0), corrosionis increased with an increase in the amount of dissolved oxygen sinceprotective films of Fe(OI-l) are not formed.

The rate of corrosion is increased with an increase in the saltconcentration of the sea water, and the maximum rate is observed atabout 0.5 N as seen in FIG. 2. The rate is decreased at the largerconcentration because of the smaller solubility of oxygen in the water.On the other hand, carbon dioxide is dissolved in sea water in the formof free CO HCO; and C0 among which free CO is the most corrosive.However, as seen in FIG. 3, the corrosion due to CO is slight relativeto the corrosion due to dissolved oxygen.

When calcium bicarbonate exists in the sea water, corrosion preventingfilm is formed to prevent corrosion. Carbon dioxide dissolved in seawater exists in the form of free carbon dioxide when the pH is below 8,while in the form of carbonate when the pH is above 8.

In conclusion, the main factors which are the most important in thepresent invention with respect to corrosion are percentage of removedcalcium, pH, the dis solved oxygen, the free carbon dioxide and thecondition under which the sea water is introduced into the flashevaporation apparatus.

The present invention has succeeded in removing the factors to favoringcorrosion so that satisfactory prevention against corrosion can beattained. As a result, the present invention has permitted the use ofwelded and drawn steel materials as well as relatively expensivematerials as heat conductive materials and construction materials for avariety of apparatus without any fear of corrosion.

in this invention, 90 to 95 percent of calcium sulfate dissolved in theoriginal sea water is reacted with magnesium carbonate, precipitated andfiltered off as calcium carbonate, and then the treated sea water isadjusted to a pH 5.0 to 5.5 with sulfuric or hydrochloric acid. A largepart of calcium carbonate is removed throughout the process, butapproximately 2 to 5 percent of Ca ion remains in the water. The Capresent in the solution is considered to be in the form of Ca(l-ICOSubsequently the clear sea water from which calcium has been removed andof which the pH has been adjusted is introduced into the deaerationtower to deaerate up to about to percent. The pH is raised during thedeaeration process to 6.0 to 6.5.

The treated sea water is then introduced into the low temperature partbelow, i.e., 80C of the flash evaporation apparatus. This would be thestage higher than the 18th, for example, in a flash evaporationapparatus containing 20 stages. By being introduced into a stage below80C, the pH of the liquid in the flash is lowered to 7.0 to 7.5.

It is important in this process that the treated sea water is introducedinto the evaporation part of the flash evaporation apparatus which is ata temperature below 80C, when the pH of the liquid in the flash becomes7.0 to 7.5. The reaction taking place is represented by the followingformula,

Ca(HCO CaCO H O CO and the CO gas produced here immediately escapes outof the flash evaporation apparatus system due to the low temperature sothat corrosion of the apparatus, if any, is negligible.

lft s satsdjsa x tsris istq ai l fq sxsmal into the heat conduction partof the 16th stage of the heat recovery part as is the case with theusual process shown in FIG. 4, the above reaction takes place with theevolution of heat in the closed heat conduction pipe and the water istransferred to the heat exchanger, where the following reactions occur:

The CO gas can not escape from the closed heat conduction pipe where itwas produced. This is a main contributor to corrosion. Since the partialpressure of C0, is high, Ca(I-ICO is transferred to the highertemperature part without being decomposed. As a result the pH of thesolution in the heat conduction pipe remains at 6.0 to 6.5, the samepI-I as it was when the solution was lead to this part. Therefore theatmosphere automatically holds the H of the solution con taining the COto 6.0 to 6.5. This means accelerated corrosion of iron and steel as isevident from the above description. The soft scale consisting ofMg(Ol-l) which was formed at the same time is apt to cause the scaletrouble when it adheres to the heat conduction pipe.

On the contrary, however, when the treated sea water is introduced atfirst into the low temperature part where the temperature is below 80Cas is the case with the process of this invention, carbonate of Ca whichremained in a small amount in the preceding stage of process isdecomposed into CaCO by evaporation, deaeration and the temperaturerise. It is supposed that a fraction thereof is consumed due to the riseof pH of the solution from (6.0 to 6.5) to (7.0 to

7.5) and the remainder forms a thin coating film to protect the steelmaterials against corrosion.

The coating film is so thin that it can hardly noticed with the nakedeye even after a long period of operation and almost no effect isobserved on the thermal conductivity. Again, this film grows during along period of operation by an unknown reason.

The present invention will be explained in detail by referring toexamples.

The example which follows is for illustrating the present inventionwhich was carried out using the apparatus shown in FIG. 5. The flashevaporation apparatus consisted of 20 stages, each being 16m long, 2mwide and 2m high, and the heat conduction area being m for each stageand 200m in total. The heat conduction tubes were made ofjointless steelpipes 27mm in diameter and 1.5mm thick and the body of the tank was madeof 6mm thick steel plate for construction, JIS SS 41.

As the brine heater, a multipipe system was used which consisted ofjointless steel pipes of 50mm inner diameter, 2mm thickness and 32m areafor heat conduction. The following is an example in which sea water wasconverted into plain water with the flash evaporation apparatus.

The original sea water was continuously introduced into the reactionvessel (1) in the rate of 4 kl/hr. A slurry of magnesium carbonate wasadded in a continuous stream to react with calcium sulfate dissolved inthe original sea water and the mixture was stirred to accelerate thereaction. The formed CaCO was lead to the thickner (2) to precipitatethe major fraction of it and the solution overflowing the thickner wasfiltered with the sand filter (3) to obtain clear sea water. The pH ofthe clear sea water was 8.5. The removed calcium sulfate amounted to 95percent of the originally dissolved calcium sulfate. The clear sea waterwas then transferred to the pH adjusting tank (4) and sulfuric acid wasadded to make the pH 5.5. The resulting solution was introduced into thedeaeration tower (5) to deaerate in vacuum with a rate of deaeration of95 percent. The degree of vacuum was 60 Torr and the temperature was 40Cin the deaeration tower, and the sea water after the deaerationtreatment, of which pH was 6.5, contained 0.33 ppm of dissolved oxygen(6.5 ppm for the original sea water). This was introduced into the stageNo. 18 (the temperature was 60C) of the flash evaporation apparatus (F)and the flash evaporation was continued. The obtained B. brine amountedto 0.82 kl/hr and the plain water to 3.18 kl/hr. The Fe content of thebrine remained always below 0.1 ppm by analysis and that of the plainwater was below 0.05 ppm. In the meantime the Cl content of the plainwater was less than 20 ppm. The corrosion of the steel material wasentirely negligible as described above. The concentration of theoriginal sea water was 3.5B. The temperature at the outlet of the brineheater (7) was 120C and the brine obtained at the stage No. 20 was 38C.

The whole apparatus was checked after continuous operation for 380 days(8160 hours), but no trace of corrosion was observed on the tank bodyand the brine heater as well as on the heat conducting pipes. They werealmost as perfect as they were at the time of construction. In otherwords, no troubles due to either corrosion or scales occurred.

Examples carried out with the mentioned apparatus are shown in Table 1along with examples for comparison.

TABLE 1 pH adjusted to 5.0

pH Fe Removed Rate of after pH in content Extent calcium deaeradeaeratheof the brine of tion tion tank 15B. scaling (p 80 5.60 6.50 2.00considerable 80 5.65 6.55 1.80 considerable 80 5.70 6.70 1.70considerable 80 5.70 6.70 1.60 considerable 85 80 5.70 6.70 2.00 medium85 85 5.70 6.70 1.80 medium 85 90 5.75 6.75 1.70 medium 85 95 5.75 6.751.50 medium 90 80 5.85 6.85 1.80 not observed 90 85 5.90 6.90 1.70 notobserved 90 90 5.90 6.90 1.50 not observed 90 95 6.00 7.00 0.50 notobserved 95 80 5.90 6.90 1.00 not observed 95 85 5.90 6.90 1.00 notobserved 95 90 5.95 6.95 0.70 not observed 95 95 6.00 7.00 0.50 notobserved pH adjusted to 5.5

pH Fe Removed Rate of after pH in content Extent calcium deaeradeaeratheof the brine of tion tion tank 15B. scaling (p 80 80 6.10 7.00 1.50considerable 80 85 6.15 7.05 1.30 do. 80 90 6.20 7.20 1.20 do. 80 956.20 7.20 0.50 do. 85 80 6.20 7.20 1.00 medium 85 85 6.20 7.20 0.75 do.85 90 6.25 7.25 0.70 do. 85 95 6.25 7.25 0.50 do. 90 80 6.35 7.35 0.90not observed 90 85 6.40 7.40 0.80 do. 90 90 6.40 7.40 0.60 do. 90 956.50 7.50 0.30 do. 95 80 6.40 7.40 0.50 not observed 95 85 6.40 7.400.40 do. 95 90 6.45 7.45 0.30 do. 95 95 6.50 7.50 0.10 do. In the nextplace examples for comparison are shown in which the pH was adjustedafter the removal of calcium outside the range specified in the presentinvention using the same apparatus.

TABLE 2 pH adjusted to 4.5 Removed calcium pH after pH in Fe content ofdeaeration the the formed 15B. tank brine (ppm) 80 5.5 6.5 3.00 85 5.56.5 3.00 90 5.5 6.5 3.00 95 5.5 6.5 3.00

pH adjusted to 6.0 Removed calcium pH after pH in Fe content ofdeaeration the the formed 15B. tank brine (ppm) 80 7.0 8.0 2.00 85 7.08.0 2.00 90 7.0 8.0 l.00 95 7.0 8.0 1.00

Subsequently examples are shown for the sake of comparison in which thesea water was introduced after the deaeration process into the flashevaporation apparatus under a different condition from that specified inthe present invention.

Sea water after the deaeration treatment was introduced in the manner asdescribed above into stage No. l6 in FIG. 4, of which the temperaturewas 88C. In this case sea water having a low pH was introduced directlyinto the heat conducting pipes and was kept in a closed state until thetemperature was elevated up to about 120C. Thus, deaeration by means ofevaporation did not take place and the pH was lower by about C than thatof the liquid in the tank.

Since the introduced sea water was evaporated at a high temperature,unfavorable precipitation of l\/lg(OH) was encountered which caused ascale problem. in this case a coating film of CaCO for preventingcorrosion was considered not to have been formed, and the CO gas whichwas produced simultaneously passed all the way throughout the entireapparatus, so that resulted in the corrosion of the materials. As aresult, the iron content of the formed brine was actually increased.Results of this example are shown in Table 3 for comparison.

TABLE 3 pH adjusted to 5.0 Removed calcium pH after pH in Fe content ofdeaeration the the formed tank B. brine (PP 80 6.0 7.0 3.50 85 6.0 7.03.50 90 6.0 7.0 3.50 95 6.0 7.0 3.00

pH adjusted to 5.5 Removed calcium pH after pH in Fe content ofdeaeration the the formed 15B. tank brine (ppm) 80 6.5 7.5 2.00 85 6.57.5 2.00 90 6.5 7.5 1.50 95 6.5 7.5 1.30

pH adjusted to 4.5 Removed calcium pH after pH in Fe content ofdeaeration the the formed tank 15B. brine (p 80 5.5 6.5 4.00 85 5.5 6.54.00 90 5.5 6.5 4.00 95 5.5 6.5 4.00

pH adjusted to 6.0 Removed calcium pH after pH in Fe content of (9%)deaeration the the formed tank 15B. brine (W Turbidity appeared in thetank and scale of Mg(OH) adhered on the glass window in the tank.

As has been mentioned above, the process of this invention permits theuse of welded and drawn steel materials almost without any precautionbeing taken against corrosion which would normally have been consideredinevitable and provides extremely excellent result in practice byevaporating specifically treated sea water with a flash evaporator undera specified condition.

Of course, various relatively expensive alloy steels which have beenemployed as corrosion resistant steel materials can be used withoutlimitation. Moreover, they can be used with more benefit for a farlonger period than generally recognized.

In addition, the process of this invention may be applied to effectivelyprevent corrosion not only to the flash evaporation apparatus itself butalso to a variety of materials attached to the flash evaporationapparatus which are involved after the deaeration apparatus, andordinary welded pipes such as a gas pipe may be employed as pipingmaterial without any precautions being taken against corrosion What isclaimed is:

l. A process for evaporating sea water without causing corrosion of thesteel materials of the apparatus comprising independently of anevaporator, adding magnesium carbonate to the sea water in an amount toreact calcium sulfate dissolved in the sea water with magnesiumcarbonate to such an extent that 5 to 10 percent of the original Caremains dissolved in solution while forming a calcium carbonateprecipitate, removing said precipitate by filtration, adjusting the pHof the treated sea water with a mineral acid to pH 5.0 to 5 .5,deaerating said treated sea water at a rate of deaeration exceeding 90percent and then introducing the sea water into a stage of a flashevaporation apparatus wherein the temperature is lower than 80C.

2. A process according to claim l in which the reaction of calciumsulfate dissolved in the sea water with magnesium is carried out at atemperature above 40C and wherein the molar ratio of CaSO. to MgCO islarger than 1 1.

3. A process according to claim l in which the mineral acid is selectedfrom the group consisting of hydrochloric acid and sulfuric acid.

0. A process according to claim 1 in which the deaeration is carried outata rate between 90 to 95 percent at a temperature between 45 to 55C.

=1 4: t= =l l=

2. A process according to claim 1 in which the reaction of calciumsulfate dissolved in the sea water with magnesium is carried out at atemperature above 40*C and wherein the molar ratio of CaSO4 to MgCO3 islarger than 1 :
 1. 3. A process according to claim 1 in which themineral acid is selected from the group consisting of hydrochloric acidand sulfuric acid.
 4. A process according to claim 1 in which thedeaeration is carried out at a rate between 90 to 95 percent at atemperature between 45* to 55*C.